Schistosomiasis, caused by parasitic flatworms living in the mesenteric/vesical veins, is the second most important infectious disease of man. Producing chronic illness, it spreads with increasing demands for water and intensified irrigation in developing countries. It is a major contributor to the high level of morbidity and mortality in endemic areas. Mollusciciding, sanitary engineering and chemotherapy have not been able to keep the disease in check. Additional control measures are needed. One such approach involves biological interference with disease transmission in the field. To some extent interference with transmission occurs naturally: by predation of cercariae, miracidia and snails by certain, fish, prawns, annelids, turbellaria and carnivorous plants. A new approach to reducing transmission depends on immobilization of cercariae, the larval which infects man by penetrating intact skin. In the laboratory a watersoluble factor emitted by a symbiotic rotifer species colonizing vector snails does this by paralyzing the cercariae and thus reducing their capacity to emerge from snails and to infect man. Phase I objectives are to: fractionate this anticercarial rotifer factor in water from cloned cultures of rotifer; isolate and purify the active fraction(s); and characterize and define it chemically and structurally.